234' Prof. Forbes on the Uefraction and Polarization of Heat. 



the pressure of the eyelid was always essential to the success 

 of the experiment. 



I have only further to add, that I should be glad to have 

 the opinions of those who are interested in the physiology of 

 vision on this curious fact, as I find it exceedingly hard to 

 conceive how such a change in the lens as I have supposed, 

 could be produced by any pressure, however exercised, of the 

 eyelid. 



Pallas Kenry. Feb. 1, 1835. D. Griffin. 



XLVIII. On the Refraction and Polarization of Heat. By 

 James D. Forbes, Esq., F.R.SS. L. Sr E., Professor of Na- 

 tural Philosophy in the University of Edinburgh. 



[Continued from p. 214.] 



§. 4?. On the Depolarization and Double Refraction of Heat 

 by Crystals. 



46. T^HE analogies which have hitherto guided us from the 

 -*• laws of light to those of heat, suggest that it is far from 

 improbable that the influence of crystallized bodies upon pola- 

 rized light, which produces the most splendid and most varied, 

 but, at the same time, amongst the most determinate phaenomena 

 of optics, may have a counterpart in the science of heat. The 

 simpler of these, of course, it is our. object first to verify; and, 

 to a certain extent, this is all that is necessary, in order to 

 complete the analogy of heat and light in this particular case; 

 for the conditions essential to their production in the case of 

 light, are on all hands admitted to depend on the susceptibi- 

 lity of the principle of light to undergo certain modifications 

 in certain circumstances, extremely limited in number, and 

 which then produce, as necessary consequences, all the sub- 

 sequent effects. If we find that heat undergoes the same 

 changes under the same circumstances, so far as we can detect 

 them, there is the highest probability in favour of the extended 

 analogy ; for if there be a necessary sequence in the one case, 

 it must be inferred also in the other. 



4-7. When polarized light is caused to pass 'through a cry- 

 stallized body possessing the power of double refraction, it hap- 

 pens, in a great majority of the conditions under which the 

 experiment may be made, that the light, on emerging from 

 the crystal, has undergone some change. This change may, 

 for instance, render it capable of reflection at a surface inclined 

 to the rays of light at the polarizing angle, which they were 

 incapable of doing before the crystal was interposed, or if be- 



